Usage: rasterize.js URL filename [paperwidth*paperheight|paperformat] [zoom]
  paper (pdf output) examples: "5in*7.5in", "10cm*20cm", "A4", "Letter"
  image (png/jpg output) examples: "1920px" entire page, window width 1920px
                                   "800px*600px" window, clipped to 800x600
